The presence of significant quantities of H.sub.2 S and CO.sub.2 in various "sour" industrial gaseous streams poses a persistent problem. Although various procedures have been developed to remove and recover these contaminants, most such processes are deficient, for a variety of reasons.
In one cyclic method currently attracting attention, the sour gas is contacted, preferably with a solvent-reactant system which comprises a regenerable reactant, to produce solid free sulfur which is recovered either prior or subsequent to regeneration. Suitable reactant materials include polyvalent metallic ions, such as iron, vanadium, copper, manganese, and nickel, and include polyvalent metal chelates. Preferred reactants are coordination complexes in which the polyvalent metals form chelates with specified organic acids.
In yet another process, e.g., that disclosed in U.S. Pat. No. 4,091,073, issued May 23, 1978, to Winkler, CO.sub.2 present in the gaseous stream is also removed by the use of a suitable selective absorbent.
Because these "cleanup" processes generally represent significant costs to manufacturing operations, any improvements in such processes which increase their efficiency may have great economic importance. For example, where ligands or chelates of polyvalent metal chelates are employed, degradation or decomposition of the polyvalent metal chelates represents an important cost in the process, as well as requiring measures for decomposition product bleed or removal and addition of fresh solution. Even in the case of preferred chelates such as those of 2-(n-hydroxyethyl) ethylene diamine-triacetic acid and nitrilotriacetic acid, ligand decomposition, over a period of time, requires attention to prevent build-up of decomposition products and consequent loss of efficiency. As will be recognized, the bleed from the process contains, along with the decomposition products, a considerable amount of the valuable polyvalent metal chelates. In one proposed process, as described in an application by Lutz, entitled Complexing Acid Recovery, filed even date herewith, and whose disclosure is incorporated herein by reference, a bleed stream or streams from the process are treated to recover the valuable chelating agent. The present invention represents an additional approach to recovery of the valuable chelating agent.